1. Field of the Invention
This invention pertains to tubes which intensify electromagnetic radiation images, such as X-ray, infra-red or low light level images.
The tubes concerned convert the image to a corresponding image of photoelectrons which are accelerated and focused on a receiving screen, such as a cathodoluminescent phosphor. Stray magnetic fields in the tube produced by other equipment or the earth's field bend the electron trajectories and distort the image. Shielding against stray fields is beneficial.
2. Description of the Prior Art
Magnetic shielding of devices which depend on the trajectories of free electrons is well known in the art. U.S. Pat. No. 2,797,408 issued June 25, 1957 to W. H. Greatbatch, Jr. et al describes a ferromagnetic shield for a cathode-ray tube. U.S. Pat. No. 2,234,281 issued Mar. 11, 1941 to E. Ruska discloses internal shielding in an electron microscope.
In image intensifier tubes concerned with the present invention the shielding problem is acute because the shield must have an aperture for the received photon image. Stray magnetic field leaking in through the aperture reaches the photocathode end of the tube where the electrons are going slowly and hence are most susceptible to magnetic deflection. Prior art schemes have tried to compensate for the image distortion by adding magnet coils in which the current is adjusted to balance out the stray fields. U.S. Pat. No. 3,809,889 issued May. 7, 1974 to Robert C. McBroom discloses a balancing coil scheme. These schemes are of limited benefit because when the external fields change or when the tube is moved the critical adjustment must be redone. Prior art passive shields of ferromagnetic material as shown in FIGS. 2 and 3 have had only limited success due to the patterns of field leaking in through the image aperture and distorted by the open edge of the shield, as described more fully below.